The disclosure pertains to a process for removing undesirable surface material from flat materials either in sheet or continuous form, and from narrow tubular material. In particular, the disclosure pertains to an apparatus and method for removing scale from the surfaces of processed sheet metal or metal tubing by propelling a scale removing medium, specifically, a liquid/particle slurry, against the surfaces of the material passed through the apparatus, and controlling the slurry blasting process in a manners to produce a resultant material that exhibits rust inhibitive properties.
As will be described in further detail below, the methods and apparatuses disclosed herein provide advantages over the apparatuses and methods used in the prior art. Sheet steel (a.k.a. flat roll) is by far the most common type of steel and is far more prevalent than bar or structural steel. Before sheet metal is used by manufacturers it is typically prepared by a hot rolling process. During the hot rolling process, carbon steel is heated to a temperature in excess of 1,500° F. (815° C.). The heated steel is passed through successive pairs of opposing rollers that reduce the thickness of the steel sheet. Once the hot rolling process is completed, the processed sheet metal or hot rolled steel is reduced in temperature, typically by quenching it in water, oil, or a polymer liquid, all of which are well known in the art. The processed sheet metal is then coiled for convenient storage and transportation to the ultimate user of the processed sheet metal, i.e. the manufacturers of aircraft, automobiles, home appliances, etc.
During the cooling stages of processing the hot rolled sheet metal, reactions of the sheet metal with oxygen in the air and with the moisture involved in the cooling process can result in the formation of an iron oxide layer, commonly referred to as “scale,” on the surfaces of the sheet metal. The rate at which the sheet metal is cooled, and the total temperature drop from the hot rolling process effect the amount and composition of the scale that forms on the surface during the cooling process.
In most cases, before the sheet metal can be used by the manufacturer, the surface of the sheet metal must be conditioned to provide an appropriate surface for the product being manufactured, so that the sheet metal surface can be painted or otherwise coated, for example, galvanized. The most common method of removing scale from the surface of hot rolled or processed sheet metal is a process known as “pickling and oiling.” In this process, the sheet metal, already cooled to ambient temperature following the hot rolling process, is uncoiled and pulled through a bath of hydrochloric acid to chemically remove the scale formed on the sheet metal surfaces. Following removal of the scale by the acid bath, the sheet metal is then washed, dried, and immediately “oiled” to protect the surfaces of the sheet metal from oxidation or rust. The oil provides a film layer barrier to air that shields the bare metal surfaces of the sheet metal from exposure to atmospheric air and moisture.
Virtually all flat rolled steel is pickled and oiled. Because flat rolled steel is so commonly used—it is typically used in automobiles, appliances, construction, and nearly all of our agricultural implements—pickling and oiling, either as an end result pickled product or pickled to produce other common materials such as cold roll, prepaint, galvanize, electro galvanize, etc, is also very common. To illustrate the scope of the practice, one of the largest steel producers in the world operates a very large steel mill that has 16 pickle lines each running about 90,000 monthly tons. Some estimate that there are approximately 100 pickle lines in the U.S. alone with several thousand more located abroad.
The “pickling” portion of the process is effective in removing substantially all of the oxide layer or scale from processed sheet metal. However, the “pickling” portion of the process has a number of disadvantages. For example, the acid used in the acid bath is corrosive; it is damaging to equipment, it is hazardous to people, and is an environmentally hazardous chemical which has special storage and disposal restrictions. In addition, the acid bath stage of the process requires a substantial area in the sheet metal processing facility. Pickling lines are typically about 300-500 feet long, so they take up an enormous amount of floor space in a steel mill. Their operation is also very expensive, operating at a cost of approximately $12/ton-$15/ton. A “pickling and oiling” line with a tension leveler costs approximately $18,000,000.00. Also, it is critical that the sheet metal be oiled immediately after the pickling process, because the bare metal surfaces will begin to oxidize almost immediately when exposed to the atmospheric air and moisture. Oftentimes, free ions from the acid solution (i.e., Cl−) remain on the surface of the metal after the pickling portion of the process, thereby accelerating oxidation unless oiled immediately.
Oiling is also effective in reducing oxidation of the metal as it shields the bare metal surfaces of the sheet metal from exposure to atmospheric air and moisture. However, oiling also has disadvantages. Applying and subsequently removing oil takes time and adds substantial cost both in terms of material cost of the oil product itself, and in terms of the labor to remove oil before subsequent processing of the steel. Like the pickling acid, oil is an environmentally hazardous material with special storage and disposal restrictions. Oil removal products are usually flammable and likewise require special controls for downstream users of the steel product. Also, again, it is critical that the sheet metal be oiled immediately after the pickling process, because the bare metal surfaces will begin to oxidize almost immediately when exposed to the atmospheric air and moisture.
The methods and apparatuses disclosed herein eliminate pickling lines and the need to put oil on the product after pickling. The methods and apparatuses disclosed herein produce a rust inhibitive product, whereas conventional shot blasting and other blasting techniques do not produce a resultant product with rust inhibitive properties, and thus do not replace the need for pickling and oiling. A processing line incorporating the methods and apparatuses disclosed herein avoids the many disadvantages of a pickling and oiling line. For instance, a processing line incorporating the methods and apparatuses disclosed herein is about 100 feet long, thereby saving significant space in a facility. The methods and apparatuses disclosed herein allow for recycling of many of the materials used in the process, without the use of harmful chemicals and acids. Operating costs associated with a processing line using the methods and apparatuses disclosed herein are $7/ton-$10/ton, which is significantly lower than the operating costs of approximately $12/ton-$15/ton associated with a “pickling and oiling” line. The capital cost of a typical line utilizing the methods and apparatuses disclosed herein is about $6,000,000.00, whereas the capital costs for a typical pickling line are about $18,000,000.00.